1. Field of the Invention
The present invention relates to a direct-current plasma CVD apparatus and a method for producing a diamond using the apparatus.
2. Description of the Related Art
Diamond has a wide band gap of 5.47 eV and a very high dielectric breakdown electric field intensity of 10 MV/cm. Furthermore, it has the highest thermal conductivity in materials. Therefore, if this is used for an electronic device, the device is advantageous as a high output electronic device.
Furthermore, diamond has a high drift mobility and is the most advantageous as a high speed electronic device in semiconductors in comparison of Johnson performance index.
Accordingly, diamond is said to be the ultimate semiconductor suitable for high frequency/high power electronic devices.
Therefore, a multilayer substrate in which a diamond film and the like is laminated on a substrate has attracted attention.
Now, most of the single crystal diamonds for fabrication of a diamond semiconductor are diamonds referred to as Ib type formed by a high pressure method. The Ib type diamonds contain a large amount of nitrogen impurities and can only be obtained at a size of no more than about a 5 mm square. Therefore, their utility is low.
By contrast, Chemical Vapor Deposition (CVD) method has an advantage that a diamond film of polycrystalline diamond of a large area having a diameter of about 6 inches (150 mm) can be obtained with a high purity.
However, in the CVD method, it has been conventionally difficult to perform single crystallization suitable for general electric devices. This is caused by a single crystal Si having been used as a substrate conventionally. That is, this is because Si and diamond are very different in lattice constant (mismatch between them is 52.6%) and it is very difficult to heteroepitaxially grow a diamond on a silicon substrate.
Therefore, various kinds of studies has progressed and there are reports that it is effective that Pt or Ir is formed as a ground film and then a diamond film is formed thereon by CVD method (see, for example, Y. Shintani, J. Mater. Res. 11, 2955 (1996), and K. Ohtsuka, Jpn. J. Appl. Phys. 35, L1072 (1996)).
In the current situation, the research relating to Ir has particularly progressed most. There is a method in which, first, by using a single crystal MgO as a substrate, an Ir film is heteroepitaxially grown thereon, and next by a direct-current plasma CVD method, the Ir film surface is pretreated by a bias treatment with a methane gas diluted by hydrogen, and a diamond film is grown on the Ir film. Thereby, there have been obtained diamonds having a conventional submicron size to a recent several millimeters' size. A thickness of the diamond portion is approximately several μm to 100 μm. For example, a diamond film is grown for 8 hours to obtain a thickness of about 100 μm in Maeda Shintaro and the other, the 18th diamond symposium lecture summary, pp. 10-11 (2004).